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-   -   Sheet Metal Thickness - Does it matter? (https://www.team-bhp.com/forum/road-safety/155988-sheet-metal-thickness-does-matter-12.html)

I was just reading the thread of Toyota Etios Liva getting a 4 star safety rating by NACP and found the link to this thread. Being a metallurgist, I think I can add some value to this post.

In principle, I do not agree that cars with thick metal sheet are safer than cars with thin metal sheet. Why I say so:

Firstly, there are several types of steel with different alloying elements and hence different properties. With present day technology, it is possible to make a very high strength steel, a 0.5 inch thickness sheet of which can have more strength than 2 mm thickness of another steel. So a light car might be using a costly high strength thin steel sheet while a heavier car might have a less expensive and less strong steel sheet but with a higher thickness. So overall strength of both metal sheets can be similar.

Secondly, a high strength metal sheet is not the ultimate required property for a car body. The most important property from a safety point of view that is required from a metal sheet for cars is “Toughness”. Toughness is the amount of energy absorbed by a metal before it fails. In the below diagram, toughness is the grey area under the curve.



To explain this: The above diagram shows the stress-strain curve for 3 kinds of steel. In simple terms, “Stress” is the applied force per unit area and “Strain” is the deformation due to the applied force. The grey area under the curves is the toughness of the steel.

As you see in high carbon steel, a lot of force is required to bring a small deformation so it is a very strong steel but it absorbs least energy before it fails (less grey area) so it has least toughness. We do not want this steel in our cars. A more intuitive way of looking at this is: Imagine a car made from 10 inch thick steel crashes, what do you think will happen to the car? The metal will not be deformed much but the passenger will be hurt badly due to the impact.

In medium and low carbon steel, the steel deforms significantly when force is applied and in the process of deformation, it absorbs a lot of energy (more grey area) thus minimizing the energy transfer to the cabin and the passengers. That said, the amount of energy absorbed by the metal sheet is quite less compared to the energy absorbed by the specially designed crumple zones and beams that are designed to fail at particular stresses to increase the safety of the passengers.

In cars, very low carbon steel is used since it has several advantages including easy to form into different contours and shapes, has good paint adhering and does not show bad deformations when a dent happens.

If I am right, why do the Germans use thicker steel? Here is my guess:

Germans started making cars several years ago when the technology to make thin steel sheets with high strength steel was not very developed. Just to maintain the structural integrity of the car, they had to use thick steel. As German cars became popular around the world, they became known for their solid build quality. This actually became one of their USP (Unique selling point).

I imagine metallurgists in these companies telling the management that they can use thinner steel and still achieve same strength and safety. But the marketing and branding guys telling them to shut up and not ruin the brand. A brand is made on the “feel” factor and we humans perceive the build quality of a car by its solid thick outer body. I am sure the solid thud sound of a closing door has been engineered to give that “feel” of solid build. This is their USP. They cannot give it up even if it makes the car lighter while keeping it at same safety level. Even I like the solid build “feel” of a Polo better than Etios in spite of knowing that the outer metal sheet does not really matter much.

This made a perfect case for the Japanese to come up with their own USP: fuel efficiency. They actually went a bit too far by using such thin steel that even leaning against a car can leave a small dent on the body.

As seen by NACP tests, both types of cars are equally safe if designed properly and have safety features like airbags, ABS etc. Now, it’s just a fight between the brands to convince people that they either have a better build quality or a better fuel efficiency.

https://www.rushlane.com/maruti-bale...-12201855.html

Guess this makes a good example of the point. Was reminded of the thread when I saw the news article.

Both cars protected the occupants, but the lack of sheet metal thickness in one meant that the crumble zones were called into action faster than the other?

So while sheet metal thickness may not matter in an NCAP test, it does matter in low speed crashes in my humble opinion.

For a good comparison, you need to bump a Jetta on to a Jetta from behind at same speed.
Crumple zones are not located in bumpers as far as I know.

There is a top gear test in which they bumped a Range Rover in to Renault espace, and espace was the winner.
Its on youtube as well.

It was due to Amby's and Padminis that we psychologically believed heavier cars are safer, which in fact had no actual reasoning.

I agree here's the video of the same. Metal thickness matters but so does other factors.

https://youtu.be/YbEa3C2vL7U

Not sure if you guys read "low speed" or just ignored it.

Kindly explain me crumple zone should work when a cyclist bumps into your car at 20kmph or less. All crashes don't happen at 60kmph. Most city incidents occur at much lesser speeds in India. And some of the new generation cars can't even withstand a cycle bumping into it, forget motorcycles. As I explained in an earlier post, all it took to form a big dent on my uncle's WagonR was for his son to lean on it while trying to push another object. Kindly explain how thin sheet metal is better in any of these scenarios, given design qualities remain the same.

Or take my question this way-

Safer body shell crumple design = Good at high speeds. No arguments there.
Safer body shell crumple design + Good sheet metal = Good at low speeds + Good at high speeds.
Safer body shell crumple design + Thin sheet metal = Good at high speeds + Good for FE.

Bad body shell design + Thick sheet metal = Good at low speeds + Poor at high speeds.
Bad body shell design + Thin sheet metal = Better parked inside the garage.

Thinking from the safety perspective, at which situations do thin sheet metal act better than the thicker ones? Safe =/= 60 kmph NCAP tests alone.

Perfect, but along with the "feel", VW also claims Polo has sheet metal tested to withstand impact from pebbles and similar small objects till a certain speed (forgot values now). I doubt Etios would come anywhere closer!

Absolutely spot on. If the aesthetics, get ruined on impact with pebbles or a small boy leaning or slow speed inevitable impacts in cramped parking lots, and each time I need to shell out a few thousands to get aesthetics right, its unsuitable. If aesthetics were NOT important, well, end of discussion then. Cars may be great looking: how long, or how well they are able to preserve the looks, by way of intact body shape, is also important. A car's sleek contour lines, dinged when a shuttle cork hits it, is unsightly, painful and expensive! Sheet metal thickness probably comes into picture here. IMHO I see more dings, small to medium dents and dislocated panels in the lighter cars. It looks ugly, to say the least. God forbid, that final, "testing" impact will happen only once, proving or disproving its worth that doomed day. BUT, with the small dents and dings and panel dislodged: I need to live with day in , day out! Unacceptable!

I think one of the important things we often forget is that weight reduction by selecting lighter but tougher materials at the same time not compromising on the structural integrity is great.

I am all game for it.

However Is it the same in case of India? I mean we have seen how pathetically the swift's structure behaves during NCAP tests. The ones sold outside on the other hand are up there with the best. It doesn't take much to see that the structural integrity is compromised here in favour of higher mileage and of course cost cutting too.

I have owned a swift and I have mentioned it elsewhere on the forum before, that build quality is pathetic. Now in the absence of definitive crash tests I would believe that the swift is as safe or unsafe as the next European heavyweights around (OK not the polo. That had among the best performing cabin in the tests among Indian Cars). However when a santro had backed onto my swift in Delhi, I had seen how an impact that did not even deform the Santro's bumper resulted in my cars bumper getting dislodged. Even in the baleno vs jetta crash I cannot help but wonder whether the Baleno's crumple zones really needed to get crumpled so much. I mean we do not even have a dislodged bumper on the Jetta.

If that is the case here, will this car provide ample protection to the cabin during a high speed crash? Or will it just crumple so quickly that all the impact will in effect be transfered to the body shell anyway?

I don't know the answer to this. I guess I will have to wait till we have definitive crash test results of all cars sold here in India.
Till then I am happy behind the 'percieved' safety of cars with thicker sheet metal (do remember these are also modern cars with well defined crumple zones and not the age old behemoths that I am referring to). I am also happy that small nicks and nudges don't require me to visit the body shop to get the denting done every now and then.